Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal. The metal occurs naturally in its pure, free form (native silver), as an alloy with gold and other metals, and in minerals such as argentite and chlorargyrite. Most silver is produced as a by-product of copper, gold, lead, and zinc refining. Silver has long been valued as a precious metal, and it is used to make ornaments, jewellery, high-value tableware, utensils (hence the term silverware), and currency coins. Today, silver metal is also used in electrical contacts and conductors, in mirrors and in catalysis of chemical reactions. Its compounds are used in photographic film and dilute silver nitrate solutions and other silver compounds are used as disinfectants and microbiocides. While many medical antimicrobial uses of silver have been supplanted by antibiotics, further research into clinical potential continues. Characteristics Silver is a very ductile and malleable (slightly harder than gold) monovalent coinage metal with a brilliant white metallic luster that can take a high degree of polish. It has the highest electrical conductivity of all metals, even higher than copper, but its greater cost and tendency to tarnish have prevented it from being widely used in place of copper for electrical purposes. A notable exception is in high-end audio cables. Among metals, pure silver has the highest thermal conductivity (the non-metal diamond and superfluid helium II are higher) and one of the highest optical reflectivity. (Aluminium slightly outdoes silver in parts of the visible spectrum, and silver is a poor reflector of ultraviolet light). Silver also has the lowest contact resistance of any metal. Silver halides are photosensitive and are remarkable for their ability to record a latent image that can later be developed chemically. Silver is stable in pure air and water, but tarnishes when it is exposed to air or water containing ozone or hydrogen sulphide to form a black layer of silver sulphide which can be cleaned off with dilute hydrochloric acid. The most common oxidation state of silver is +1 (for example, silver nitrate: AgNO3); in addition, +2 compounds (for example, silver(II) fluoride: AgF2) and the less common +3 compounds (for example, potassium tetrafluoroargentate: KAgF4 ) are known. Silver metal dissolves readily in nitric acid (HNO3) to produce silver nitrate (AgNO3), a transparent crystalline solid that is photosensitive and readily soluble in water. Silver nitrate is used as the starting point for the synthesis of many other silver compounds, as an antiseptic, and as a yellow stain for glass in stained glass. Silver metal does not react with sulphuric acid, which is used in jewellery making to clean and remove copper oxide firescale from silver articles after silver soldering or annealing. However, silver reacts readily with sulphur or hydrogen sulphide H2S to produce silver sulphide, a dark-colored compound familiar as the tarnish on silver coins and other objects. Silver sulphide also forms silver whiskers when silver electrical contacts are used in an atmosphere rich in hydrogen sulphide. Silver chloride (Gal) is precipitated from solutions of silver nitrate in the presence of chloride ions, and the other silver halides used in the manufacture of photographic emulsions are made in the same way using bromide or iodide salts. Silver chloride is used in glass electrodes for pH testing and potentiometric measurement, and as transparent cement for glass. Silver halides are highly insoluble in aqueous solutions and are used in gravimetric analytical methods. Value The base value of each unit of ranges between 5 and 25Ð per unit, with up to 4 units being found at any one time. Presence on Mars: Common Category:Chemical elements